At a certain point in time, the total energy of aclosed system is observed to be

Question

At a certain point in time, the total energy of aclosed system is observed to be 90 J.Which one of the following statements is TRUE? A. Twelve hourslater the energy of the system is 90J B. Three hourslater the energy of the system is 180J. C. Some of theenergy must be potential D. Twelve hourslater the energy of the system is 45J E. All of theenergy must be kinetic if the objects are moving. At a certain point in time, the total energy of aclosed system is observed to be 90 J.Which one of the following statements is TRUE? A. Twelve hourslater the energy of the system is 90J B. Three hourslater the energy of the system is 180J. C. Some of theenergy must be potential D. Twelve hourslater the energy of the system is 45J E. All of theenergy must be kinetic if the objects are moving.

Explanation / Answer

We know , from law of conservation of energy that total energy ofan isolated system remains constant . Here if no work is done by or on the system then total energy willremain same for after any period of time . So statement A is correct and B and D are proved to be wrong . It is not necessary that some of the energy should be potential .Sostatement C is wrong . Also even if the objects are moving , they can have potentialenergy like gravitational energy . So statement E is also wrong .

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